The invention concerns a method for producing a bristle from a thermoplastic polymer through injection molding, wherein the molten polymer mass is injected under pressure into a bristle-molding channel of predetermined length having a predetermined cross-sectional shape along this length and the channel is vented during injection molding. The invention also concerns a device for carrying out this method.
Animal hair and natural fibers which were previously used as bristle material for producing brushes, paint brushes or the like have been substantially replaced by artificial bristles, wherein the production of the bristle material is based largely on long-standing technology related to the production of synthetic textile fibers, i.e. extrusion or spinning processes. However, a bristle is subjected to completely different conditions than an endless fiber in a fiber composite. It is free and fixed at only one end and can be regarded in terms of stability as a bar which bends and which is fixed at one end. Pressure or compression forces and sometimes also tensile forces occur during use. Compared to endless fibers, the production requirements are different with regard to bending strength, fatigue strength under reversed bending stresses, buckling resistance and bend recovery.
Monofilaments for bristles are therefore extruded having relatively large diameters up to a few millimeters. Shaping by the extrusion and spinning nozzle produces a certain longitudinal orientation of the molecules in the molten polymer mass which is however not sufficient to provide the monofilament with the desired properties. The monofilament is therefore drawn, i.e. stretched under appropriate drawing forces, which usually requires pre-drawing, post-drawing, and subsequently thermal stabilization, which can be repeated, if required. The endless monofilament is subsequently wound up and the wound-up product is again stabilized, if required.
If, for production of brushes, the endless monofilaments are not processed directly from the spool—which is still the exception today—a large number of monofilaments are combined into strands and bound and cut to suitable lengths of between 60 and 120 cm. The strand material is again cut to a length, which is slightly longer than the final bristles thereby producing waste of approximately 30% of the initial material. For high-quality plastic bristles, e.g. of polyamides (nylon), which are required for quality brushes, e.g. toothbrushes, hygiene brushes etc., the price for the raw material is the most expensive factor in the brush price. The price of extruded bristles is consequently considerably increased by the large amount of waste.
For brushes, the production of bristles is followed by mounting the bristles to the bristle support, which can be effected either mechanically or thermally. Since the free length of the bristles largely varies in this intermediate state, shearing-off and in most cases post-processing of the bristles and mainly of the bristle ends follows to remove the sharp cutting edges. If the effective brushing surface formed by the free ends must meet special requirements, e.g. for toothbrushes, the brushing surface must either be given a contour already during mounting or the flat brushing surface must be subsequently shaped, which produces additional waste of approximately 10%.
Considering the fact that approximately 90% of the worldwide need of bristles is limited to bristles having a length of <10 cm, the endless production through spinning including all subsequent work processes until the bristle is finished is highly uneconomical, due to the raw material waste alone. Further limitations result from the fact that monofilaments can usually only be produced with cylindrical shape and with profiled cross-section such that the structure of the bristles is limited and extensive later processing may be required.
Injection molding production of brush bodies, brush handles, paint brush handles etc. from plastic material was established quite early in the brush and paint brush industry to utilize the numerous structural possibilities of injection molding technology. Various attempts were made to produce the brush body with integral bristles through injection molding. In practice, these methods are used only for bristles of the lowest quality and stability requirements, in particular those which are used only once or a few times. Injected bristles have a much worse bending strength, fatigue strength under reversed bending stresses and buckling strength, insufficient bend recovery and low wear resistance. Injected brushes have highly conical bristles with relatively large cross-sections in the root region of the bristle due to the method, and are therefore more appropriately described as pins or bolts rather than bristles. Some known injection molding methods in brush technology are described below.
Rotating bristles for grinding and polishing surfaces are composed of disc-shaped brush segments, which are produced individually through injection molding (U.S. Pat. No. 5,903,951). Each brush segment comprises a central support disc from which the bristles outwardly extend radially or at an angle inclined against the direction of rotation relative to the radial direction. The brush segments consist of a thermoplastic or thermoelastic polymer (TP or TPE), which is filled with abrasive particles. The bristles preferably have a length of between 1 cm and 5 cm and a diameter of between 0.25 mm and 10 mm, preferably between 1 mm and 2 mm. In one concrete embodiment, the conical bristles have a length of 75 mm and a diameter of 2 mm at the root and 1.5 mm at the tip. The two-part injection mold consists of two plates having the cavities for the support disc and the bristles on mutually facing sides, which simultaneously form the mold-separating plane. The molten polymer mass with the admixed abrasive particles is injected from the center of the support disc at an injection pressure of 690 to 6900 kPa (0.59 to 69 bar). The preferred pressure range is between 2070 to 4830 kPa. The required venting of the mold cavity occurs in the mold-separating plane, i.e. parallel to the bristles. This unavoidably produces two mold-separating seams on the bristle jacket, which extend from the root to beyond the tip. The abrasive particles cause additional narrowing of the small cross-sections in the bristle cavities and the molten polymer mass solidifies too quickly at these locations prior to complete filling of the bristle cavity. For this reason, injection molding is preferred in two steps, wherein a highly filled molten polymer mass is initially injected into the bristle cavities and a more or less unfilled molten polymer mass is then subsequently injected. One of average skill in the art knows that during injection molding, practically no molecular orientation takes place in the polymer (US 2001/0007161 A1, see column 1, paragraph 0006). This produces a completely insufficient bending behavior for bristles, which is additionally deteriorated by the admixed abrasive particles. The stated maximum injection pressure of 6900 kPa (69 bar) is strongly reduced through the flow resistance in the narrow mold cavity for forming the carrier disc and in the subsequent bristle channels such that the person skilled in the art may have reasonable doubts about the practicability of this method.
U.S. Pat. No. 3,618,154 describes the production of a toothbrush in one single injection molding process wherein the bristles on the brush head are injected in a type of bundle arrangement. Towards this end, the two-part injection molding tool whose mold-separating plane is in the plane of the bristle head, has substantially cylindrical bores which extend from the mold surface forming the bristle side of the brush head. Substantially cylindrical mold cores engage in the bores from the opposite side wherein one of their end faces forms part of the mold surface for the bristle support side of the head and—starting therefrom—comprise groove-like depressions which extend along jacket lines. These groove-like depressions taper uniformly and conically from the front-side mold surface towards the other end and terminate in a semi-spherical dome on the jacket of the mold core on which the depressions are uniformly distributed. Each depression forms, together with the bore wall in the one part of the injection mold, a bristle-molding channel, which consequently conically tapers from the mold cavity for the brush head towards the other end. The channels are vented across their entire length in the separating surface between mold core and bore, i.e. substantially parallel to the bristles. U.S. Pat. No. 3,618,154 requires high precision of the cooperating surfaces. Each bristle inevitably has two mold-separating seams, which extend along jacket lines on the bristle. It is also not possible to produce bristles with circular cross-section since the groove-like depression in the mold core has a substantially larger radius of curvature than the bore. This produces a cross-sectional shape with discontinuities at which the mold-separating seams, which cannot be subsequently removed, immediately form. The bending behavior of the bristle differs in different directions transverse to its axis. Furthermore, the bundles are not filled (their center is free) so that the bristles cannot support each other as is the case in conventional bundles. The serious problem of removing the individual bristles from the mold is intended to be solved through corresponding conicity of the bristle-forming grooves. This can obviously not work, since the mold cores are simultaneously used as ejector pins which push towards the bristle tips during release from the mold via the dome-shaped ends of the groove-like depressions. The conicity is intended to make the bristle ends relatively flexible during use of the toothbrush. This document does not describe any measures which extend beyond conventional injection molding technology and which could improve the bending behavior of the injection-molded bristles. In this case as well, the polymer molecules, as is usual in injection molding, have the energetically favorable balled shape, which is, however, unfavorable with regard to stability (US 2001/0007161 A1).
Moreover, in conventional toothbrush production (U.S. Pat. No. 5,158,342) the bristle stock is subsequently injected into a prepared depression of the brush head of a pre-injected brush body, consisting of handle and the brush head. This produces bristles of completely insufficient bending behavior due to the conventional injection molding technology with injection pressures of 30 to 60 bar (3000 to 6000 kPa).
GB 2 151 971 also describes two-step production of bristle stock and a bristle support. In particular, this document clearly illustrates the problem of releasing the bristles from the bristle-molding channels. Despite the strong conicity of the bristles, which is favorable for release from the mold, the mold removal process is slow and highly controlled, which impairs the efficiency of the injection molding system. Injection molding measures to increase the bristle stability are not described.
Much better results are obtained according to an older, not pre-published patent application of the inventor (PCT/EP01/07439) with which a bristle support is provided with bores which have a nozzle-like cross-sectional shape. The molten polymer mass for the bristles is injected through the nozzle-like bores into adjoining molding channels of an injection mold. This method produces a semi-finished product from bristle support and bristles or also—with corresponding shape of the bristle support—a finished brush, wherein the bristles have bending behavior characteristics, which are similar in quality to those of extruded, bristles. The shape of the bristles is not subjected to the constraints of endless production of extruded monofilaments.
U.S. Pat. No. 4,712,936 discloses production of small application brushes, e.g. for decorative cosmetics, which are inserted into a container and mounted to the sealing cap of this container, as one-part injection molding part which consists of cap, a stem centrically adjoining the inner side thereof, and brush bristles disposed at the end thereof. The mold cavities for the cap, the centrical stem and the joining bristle-molding channels are formed in the two parts of an injection molding tool with axial orientation, wherein the cap opening is in the mold separating plane thereof. The stem and bristles are produced through mold cores, which are pushed coaxially into each other. The injection side is on the cap. The molten polymer mass must consequently traverse long flow paths with several cross-sectional changes and overcome large mass requirements before reaching the thin bristle channels. The entire venting of the stem and bristle region takes place at the ends of the bristle channels via a cylindrical closure with knurl structure which is to form a type of filter with high flow resistance. This prior art shows that the bristles that can be produced through injection molding are not suitable, in particular, for use as paintbrushes. After removal from the mold, they are therefore re-heated outside of the injection mold and are subsequently drawn. The cross-section is thereby reduced which inevitably increases the separation between the bristles. However, for application brushes of this type, the bristles should be disposed at minimum mutual separations to produce capillary action between the bristles for storing and retaining the application medium.
Attempts have also been made (DE 21 55 888 C3) to produce a brush with formed-on bristles through injection molding with an injection molding tool having a first tool part for the bristle support and a second tool part largely covering the open mold cavity in which a short channel is formed which widens at its opposite end and is closed at that location. During injection, the molten polymer mass penetrates from the mold cavity of the support into the short channel and flows into the widening to produce a short bolt with a head. During opening of the mold, the head is carried along and the bolt-like bristle blank is drawn. This can produce a certain molecular orientation, which increases the stability—similar to production of endless monofilaments.
Attempts to replace production of bristles from extruded endless monofilaments and their subsequent mounting to separately produced brush bodies, with an injection molding of the entire brush with bristles have therefore obviously failed (US 2001/0007161 A1).
This is also true for the known suggestion of only producing the bristle through injection molding (U.S. Pat. No. 3,256,545). This closest prior art is based on the realization that extruded bristles have ends of increased flexibility imparted to them by post-processing of the bristle ends as do bristles obtained through injection molding of one-piece brushes in consequence of the conicity required for injection molding, which have however disadvantageous effects with regard to wear resistance and durability. This patent method suggests improving the wear resistance, which decreases towards the ends, by enlarging the cross-section of the injected bristle, going from the end on the mounting side (the injection side bristle root) towards the free end. The cross-sectional shape may increase continuously or discontinuously. In any event, a larger amount of plastic material is present in the region of the working ends of the bristles than on the mounting side end. The insufficient properties of known conical bristles are compensated for through accumulation of a larger plastic mass in the region of the bristle ends. However, one has thereby overlooked the fact that, as the plastic mass or cross-section increases, the proportion of the energetically favorable balled structure increases, i.e. the bristle excessively looses bending elasticity due to the enlarged cross-section. This injection molding method proposes injection pressures of between 800 and 1200 bar (approximately 0.8·105 to 1.2·105 kPa), which are required to introduce the molten polymer mass through the channels, which are initially narrower on the injection side, into the extended channels such that they fill the mold. Despite the relatively high pressure, the recommended bristle diameters of unoriented molecular structures are between 1.6 and 2.2 mm in the region of the thinner cross-section and between 11 and 12 mm in the region of the thicker cross-section (column 5, lines 43 to 48 and column, lines 32 to 42). Support structures of the same molten polymer mass are formed on the injection side of the bristles, for mounting the injection-molded bristles to a bristle support, and interconnect several bristles, if required.
The technical literature also teaches (Ehrenstein: Eingenverstärkung von Thermoplasten im Schmelze-Deformationsprozeβ in the German magazine “Die Angewandte Makromolekulare Chemie” 175 (1990), pages 187 to 203) that for polyamides, only 3% and 6% and for polyethylene only 33% and 5.5% of the theoretical mechanical values for the modulus of elasticity [N/mm2] and tensile strength [N/mm2] respectively are obtained through extrusion and injection molding methods, wherein for injection-molded components, the tension-free state (molecular balled structure) is preferred.
It is the underlying object of the invention to produce bristles through injecting molding whose bending behavior and bend recovery is superior to that of extruded bristles, and which permits maximum attainment of the theoretical values of the modulus of elasticity and tensile strength to produce bristles of high quality through a large length range with relatively small cross-sections for simplified production of bristle geometries and bristle arrangements adjusted to the requirements of the final product such as brushes or paint brushes. The invention also concerns a device, which is suitable for carrying out the method.